Magnetic Resonance Imaging (MRI) is a medical technique for creating cross-sectional images of the body which are then used to diagnose injury or disease. A typical MRI system will use powerful radio waves and cylindrical magnets to manipulate the natural magnetic properties of the human body. Essentially, an MRI system monitors resonant movements of hydrogen atoms as they are alternately magnetized and bombarded with radio waves. The state of hydrogen in diseased or injured tissue is different than in normal tissue and so they are more readily detected under a typical MRI procedure.
When patients are undergoing an MRI procedure to obtain information about their internal tissue structure, it is sometimes beneficial to simultaneously obtain electrophysiological data, to further aid in the patient's diagnosis or to monitor the patient's condition. For example, it may be beneficial to utilize electroencephalograms (EEG) or electrocardiograms (ECG) in conjunction with an MRI system. However, obtaining electrophysiological data via prior art devices may create electromagnetic interference that corrupts the MRI procedure. Alternatively, the MRI device may create electromagnetic interference which corrupts electrophysiological data transmitted via prior art approaches.
For example, modern EEG machines normally use high speed digital switching signals to acquire the EEG waveforms in a digital format suitable for viewing and processing by a computer. This requires an analog to digital converter (ADC) to transform the EEG into a binary representation, typically 8 to 24 bits, of the original analog signal. This binary signal is then readily transferred to a computer through a standard interface such as the USB, Parallel port or Ethernet port. Digital signals have very fast switching edges which can generate interfering harmonics in the tens of megahertz frequency range even though the fundamental digital signal frequency may be only a few kilohertz. A major problem is that these high frequency harmonics can radiate from equipment located near the MRI machine and interfere with the imaging process. Consequently, major sources of high frequency emissions such as computers and conventional electrophysiological records must be located outside the MRI room.
In the prior art, if the patient's EEG or ECG is to be recorded the electrode signals are sent via a long cable to the EEG machine located outside the MRI room. However, using the long cable for EEG signals allows electrical interference from the MRI system or nearby power mains to be capacitively coupled to the electrophysiological signal. The amplitude of such interference may severely degrade the EEG signal. A typical EEG signal is only 10 to 100 microvolts peak to peak and is therefore easily contaminated by main power cables which have voltages in the order of 250 to 550 volts peak to peak.
Consequently, there is a need for a method and apparatus which minimizes the amount of electromagnetic interference generated during an MRI procedure wherein a patient is concurrently being monitored.